This invention relates to the recovery of oil from subterranean petroleum reservoirs and more particularly to an improved micellar solution flooding process for enhanced petroleum recovery.
It has long been recognized that substantial amounts of oil remain unrecovered in the reservoir at the location of normal primary recovery operations. Hence, various secondary methods of recovering additional quantities of oil have been proposed, such as the well-known technique of waterflooding in which water is injected into the reservoir through an injection well to displace and drive additional oil toward one or more production wells in the reservoir spaced apart from the injection well. Although an additional quantity of oil often can be recovered by waterflooding, the efficiency of the waterflood and the ultimate oil recovery can be further improved by introducing into the reservoir ahead of the flood water a displacement fluid that is miscible with both the petroleum in the reservoir and with the flood water.
Micellar displacement fluids, which can be oleaginous or aqueous based systems, comprise solutions or dispersions of surfactants in concentrations in excess of the critical micelle concentration of the surfactant-fluid system. The micellar solutions are miscible with both water and petroleum and thus, when injected into the reservoir ahead of the waterflood, enhance the displacement of the petroleum toward one or more production wells from which the petroleum is recovered.
It is recognized in connection with the use of micellar solutions that the presence of multivalent ions, such as for example calcium, magnesium, barium, iron and the like have a deleterious effect upon the micellar solution system. If present in the reservoir in sufficient quantities these ions can ultimately effect a breakdown of the micellar solution and the resultant loss of effectiveness of the solution. To reduce the concentration of multivalent cations present in the reservoir it has been suggested in the art to utilize pre-injections or preflushes of fluids designed to wash away or to otherwise tie up the multivalent cations prior to injection of the micellar solution into the reservoir. Thus, for example, a pretreatment with water having a low concentration of multivalent cations can be used to dilute or displace the multivalent cations present in the free reservoir water.
More recently, it has been found that aqueous solutions of alkali metal having a controlled molar ratio of alkali metal oxide to silica are effective as a pretreatment solution to improve the efficiency of oil recovery utilizing micellar displacement fluids.
It has been found, however, for a variety of reasons, including differences in viscosity, that often the silicate pretreatment solution has a tendency to travel through the reservoir at somewhat a greater rate than the micellar solution, thus forming a gap between the preflush solution and the micellar solution that increases as the fluids travel away from the injection well. The gap permits undesired contact between the micellar solution and multivalent ions and accordingly the micellar solution can be adversely affected even though a silicate pretreatment solution has been used.
The present invention overcomes the foregoing deficiency in the prior art by eliminating or reducing the gap between the two solutions.